This invention relates generally to a liquid level system and, more particularly, to such a system employing a differential pressure unit (DPU) for detecting the level of a liquid in a tank.
The water level in boilers and pressurized tanks is often measured by a DPU such as disclosed in U.S. Pat. Nos. 2,632,474 and 2,664,749. The DPU is connected to upper and lower regions of the tank by external conduits. The conduit connected to the upper region of the tank contains a sensor which is often referred to in the art as being a "seal pot." the conduit extending from the sensor to the DPU is filled with a constant level of water providing a "reference column." This reference column is essentially at ambient temperature, and applies a constant hydrostatic pressure to one side of the DPU. Steam pressure in the tank is also applied through the reference column to said one side of the DPU. The other side of the DPU detects the hydrostatic pressure of the water and the steam pressure in the tank. Since the steam pressure in the tank is applied to both sides of the DPU, its effect becomes cancelled. Thus, as the water level changes, the DPU indicates the magnitude and direction of change in the level.
A shortcoming of the aforementioned system is the possible loss of water in the reference column conduit coupling the sensor to the DPU. This loss may occur because of leakage in the conduit or seals at the sensor or DPU, sudden depressurization of the tank causing "flashing," or incomplete filling of the conduit during start-up of the system. The loss of water will cause the DPU to produce faulty signals. The signals will indicate the water level in the tank is increasing, whereas the level may actually be dropping.
It is, therefore, one object of the present invention to provide an alarm device for indicating a loss of water in the liquid level measuring system.
Reference is made to the liquid level systems disclosed in my U.S. Pat. No. 3,853,006 and my copending U.S. patent application Ser. No. 176,466, filed Aug. 8, 1980, entitled "Temperature Compensation Apparatus for a Liquid Filled Conduit " which embody a feature common to one of the features of the present invention, namely, the use of a bellows in a seal pot.
The bellows in my previous systems forms the upper end of the reference column conduit. If water is lost from the conduit, the resulting high differential pressure across the bellows may cause the bellows to collapse and rupture. This may result in liquid in the tank or seal pot, which may be toxic or otherwise dangerous, leaking out from the conduit.
It is another object of the present invention to provide means for closing off the reference column in the event the bellows ruptures.